1. Field of the Invention
The present invention relates to fasteners used in eyeglass frames for holding lenses into lens frames and temple pieces to the lens frames and hinges.
2. Description of Related Art
Historically small screws have been used to attach various eyeglass frame components together to form a completed frame. Screws specifically are used to attach temple pieces at the hinge to lens frames. They are also used to tension and anchor or lock lenses into lens frames allowing assembly and disassembly for replacement of lens or damaged components or initial shipping of frames with demonstration lenses in place.
In normal use eyeglass frames are subjected to stresses and strains in a cyclical manner that tend to loosen screws and wear the threads with which the screws are mated resulting in temple pieces becoming loose or detached and lenses falling out of their frames.
Various methods to minimize this problem have been tried with limited success. Examples are polymeric compounds often referred to as "locktite" intended to at least slow down the loosening process. This method requires individual coating of the screws which is expensive and provides only limited improvement over uncoated screws eventually succumbing to the same process of loosening. Another example of a method intended to "lock" the screws in place is the use of distorted threads on the screw to create a mechanical jamming action with the mating threads. This method offers only limited improvement again eventually succumbing to the loosening process as well. Both methods make any replacement or disassembly and reassembly process difficult or impossible and do not accommodate wear of the mated components even if no loosening or backing out of the screws occurs.
U.S. Pat. No. 4,896,955 ("the '955 patent"), the subject matter of which is hereby incorporated by reference, discloses an eyeglass frame, of which components are formed from nickel-titanium based shape memory alloys. Components of the frame can be formed from such alloys treated in a variety of ways, to confer desired properties on the component. For example, fastening elements used in the frame can exhibit conventional shaped memory properties of the alloy. Other components of the frame can exhibit the enhanced elastic properties exhibited by shape memory alloys.
A shape memory alloy can exhibit "super elastic" behavior at a temperature below the M.sub.s of the alloy as a result of a significant degree of work-hardening, for example to about 30% or more plastic deformation. By selection of an alloy with an appropriate M.sub.s temperature, with appropriate work-hardening, such behavior can be obtained in a temperature range of -20.degree. C. to 40.degree. C.
A shape memory alloy can exhibit "pseudo-elastic" properties. Such properties are exhibited in a narrow temperature range, between the M.sub.s and M.sub.d temperatures of the alloy. They involve transformation of an alloy in its austenite phase to its martensite phase by the application of stress. Application of stress in these conditions can give rise to high strain values. Provided that the temperature is between the M.sub.s and the M.sub.d temperatures of the alloy, virtually all of the strain is recovered. The deformation and recovery are marked by significant deviations from linear elastic behavior.
A shape memory alloy which has been work-hardened, perhaps to 30% or more plastic deformation, and at a temperature between its M.sub.s and M.sub.d temperature, exhibits a combination of superelastic and pseudoelastic behavior known as "optimized elastic" behavior. The optimized elastic behavior is referred to in the '955 patent as "work-hardened pseudoelastic" behavior.
It is advantageous to use shape memory alloys which exhibit enhanced elastic behavior in the manufacture of eyeglass frames, for their kink resistance. Pseudoelastic properties are particularly preferred, because of the large amount of strain which can be recovered. However, a disadvantage of relying on pseudoelastic properties is that they are available only over a narrow temperature range.
This problem was addressed in the '955 patent by subjecting shape memory alloy eyeglass frame components to work-hardening. In this way, the advantages of superelastic behavior, and some of the advantages of pseudoelastic behavior can be obtained from temperatures below the M.sub.s temperature up to the M.sub.d temperature of the selected alloy.